Method and apparatus for imparting to headphones the sound-reproducing characteristics of loudspeakers

ABSTRACT

Sound signals are created at the eardrums of a listener to correspond to sound signals which would be created at the eardrums of such listener in a predetermined acoustical environment in response to first electrical signals applied to a loudspeaker having known sound-reproducing characteristics. The soundwaves at the eardrums of the listener are measured as a function of frequency to determine the pressure signals at the eardrums of the listener when the listener is listening to acoustical information transmitted by the loudspeaker. An electroacoustical transfer function, relating the electrical input signal of the loudspeaker to the pressure actually impinging upon the eardrums is determined. The same is done for a set of earphones, so as to determine a second transfer function associated with the earphones. A compensating network is connected to the earphones. The compensating network has a transfer function corresponding to the quotient of the first transfer function divided by the second transfer function.

United States Patent [191 Blauert et al.

[ Nov. 18, 1975 [54] METHOD AND APPARATUS FOR IMPARTING TO HEADPHONESTHE SOUND-REPRODUCING CHARACTERISTICS OF LOUDSPEAKERS [75] Inventors:Jens Blauert; Peter Laws, both of Aachen, Germany [73] Assignee: EugenBeyer Elektrotechnische Fabrik, Heilbronn, Germany 221 Filed: Sept. 7,1973 21 Appl. No.: 395,371

[30] Foreign Application Priority Data Sept. 8, 1972 Germany 2244162[52] US. Cl. 179/1 G [51] Int. Cl H04! 5/00 [58] Field of Search 179/1G, l D, 1 GA, 1 GP, 179/ 100.45

[56] References Cited UNITED STATES PATENTS 3,088,997 5/1963 Bauer 179/1G FOREIGN PATENTS OR APPLICATIONS 1,187,285 4/1970 United Kingdom 179/1G OTHER PUBLICATIONS Stereophonic Earphones & Binaural LoudspeakersbyBauer, Journal AES, Apr, 1961.

Primary Examiner-Kathleen H. Claffy Assistant ExaminerThomas DAmicoAttorney, Agent, or Firm-Michael S. Striker ABSTRACT Sound signals arecreated at the eardrums of a listener to correspond to sound signalswhich would be created at the eardrums ,of such listener in apredetermined acoustical environment in response to first electricalsignals applied to a loudspeaker having known sound-reproducingcharacteristics. The soundwaves at the eardrums of the listener aremeasured as a function of frequency to determine the pressure signals atthe eardrums of the listener when the listener is listening toacoustical information transmitted by the loudspeaker. Anelectroacoustical transfer function, relating the electrical inputsignal of the loudspeaker to the pressure actually impinging upon theeardrums is determined. The same is done for a set of earphones, so

as to determinea second transfer function associated with the earphones.A compensating network is connected to the earphones. The compensatingnetwork has a transfer function corresponding to the quotient of thefirst transfer function divided by the second transfer function.

5 Claims, 18 Drawing Figures U.S. Patant N0v. 18,197 5 Sheet 1 of133,920,904

US. Patent Nov. 18, 1975 Sheet2of13 3,920,904

B TRl Fig. 6

U.S. Patent Nov. 18, 1975 Shee t4 of 13 3,920,904

U.S. Patent Nov.l1s, 1975 Sheet50f13 3,920,904

m gs US. Patent N0v.18, 1975 Sheet60fl3 3,920,904

bQxQQv V US. Patent Nov. 18,1975 Sheet7of13 3,920,904

Sheet8of13 3,920,904

11.8. Patent Nov. 18, 1975 US. Patent Nov. 18, 1975 Sheet90f13 3,920,904

U.S. Patent Nov; 18, 1975 Sheet 11 of 13 3,920,904

U.S. Patent Nov. 18, 1975 Sheet 12 of 13 3,920,904

METHOD AND APPARATUS FOR IMPARTING TO HEADPHONES THE SOUND-REPRODUCINGCHARACTERISTICS OF LOUDSPEAKERS The invention relates to a method forimparting to an earphone set at least some of the sound-reproducingcharacteristics normally exhibited only by a loud speaker, or by aplurality of spaced loudspeakers in the case of stereo reproduction. Theinvention relates not only to the frequency response characteristics ofearphone sets versus loudspeakers, but is furthermore concerned with thepossibility of simulating the spaciousness of the sound reproduced by aloudspeaker, and particularly that produced by a plurality of speakersspaced apart from each other.

Earphones are being used nowadays in ever greater numbers, especially in'the entertainment industry, wherein they are used now very commonly forlistening to radio broadcasts, phonograph records and tape rej speaker)has'associated therewith a certain acoustical cordings. In addition,earphones are used for technical One of the principal reasons for thegrowing popularity of earphone sets for home use, as opposed to the morefamiliar loudspeakers, is that they permit listen- 1 ing to broadcastand recorded material without disturbing other persons not wishing tolisten, and likewise prevent the listener from being distracted by othersound sources which may be present.

A significant disadvantage associated with the use of earphone listeningsets, as opposed to ordinary loudspeakers, is that earphones, even thoseof high quality, exhibit sound-reproducing characteristics markedlydifferent from those of loudspeakers. These different sound-reproducingcharacteristics include not only differences in frequency response, butequally important, differences in the sense of acoustical spaciousnessexperienced by the listener.

In order to make clearer the concepts in question, reference should behad to FIG. 1. An acoustical event has objective temporal, spatial andspectral characteristics. The objectively determinable parameters of anacoustical event include the nature of the sound source, the distance ofthe sound source from the hearer, the orientation of the sound sourcewith respect to the hearer, the particular sound field or sound signalsimpinging upon the eardrums of the hearer, i.e., such parametersas canbe measured for purposes of an objective description of the physicalphenomena associated with the perception of an acoustical event.

FIG. 1 depicts, by way of example, a human hearer VP and a loudspeaker Lspaced some distance from the }human hearer in a room characterized byvery low reverberation, i.e., the walls, floor and ceiling of theroomtend to absorb rather than reflect the sound waves impinging upon them,so that the sound perceived by the hearer VP is not markedly complicatedby the superim position upon the source waves of manytimes reflectedsound waves.

2 importance, under certain definite experimental conditions a specificacoustical event s (here the electroacoustically converted signalemanating from the loudperception h This acoustical perception h has itsexistence in the nervous system and brain of the listener, and does notnecessarily correspond to the actual location and orientation of theacoustical event, as is clear from FIG. 1. The characteristics of theacoustical perception h can only be determined by receiving verbaldescriptions from a human hearer concerning his subjective experience ofthe acoustical event. The acoustical perception h accordingly cannot bemeasured directly,as can the acoustical event s although clearly theacoustical perception h, is of far greater importance than the abstractacoustical event's When an earphone set is plugged into the sameelectrical outputs into which are plugged the inputs of a loudspeaker orloudspeaker system, in order to run a comparison test using the samemusical composition, for example, very marked differences are observedin the acoustical perceptions of the listener, when using the earphonesinstead of the loudspeaker. Aside from the differences in frequencyresponse, there are other differencesof a psychological nature relatingto the spatial characteristics of the perceived sound, the mostimportant of which is the well-known orchestra in the head on the orderof magnitude of the distancebetween the listeners cars. This isespecially the' case when listening to loud music, which is frequentlydone when listening with high-quality stereophonic equipment.

It has been extremely difficult to deal in a systematic and scientificmanner with these psychological phenomena. The causesof these phenomenahave always beenassumed to include such factors as unavoidabledifferences in the sound-reproducing characteristics of the twotransducers of an earphone set, the exact positioning of the earpieceswith respect to the listeners ears, the pressure with which theearpieces press against the listeners ears, the sound transmis'sivity ofthe skull bone of the particular listener, the effect of the listenermoving his or her-head while listening, and many other suchpsychological and physiological factors. Also to be considered is thefact that when a listener employs an earphone setthe signals actuallyimpinging upon his eardrums are produced in a manner different from themanner in which signals are produced by a loudspeaker, especially alarge loudspeaker. In particular, when listening with earphones, thetotal electroacoustical transduction phenomenon does not include thefactors of substantial transmission distance, the sound distributionwithin the room between speaker and listener, the diffusion of soundbefore the sound reaches the listeners eardrums, etc.; instead, the

If an electrical signal is applied to the electrical input of theloudspeaker, the human hearer .VP willzexperience an acousticalperception h located, sofar as the =hearer can determine, at a distancer and oriented in the indicated direction with respect to the center ofthe listeners head, the acoustical perception appearing to be more orlesssharply localized in the graphically depicted position of FIG. 1. Inother words, and this is of German Offenlegungsschrift 1,927,401discloses an attempt to deal with this problem. According to theapproach in question, experiments are conducted on an artificiallyconstructed human head provided with two microphones in the regions ofthe ears of the artificial head. The acoustical characteristics of anactual human head are simulated to the extent possible, and measurementsare taken' of the sound reception in the eardrum locations of such head.As a result of the measurements taken, recording engineers can modifytheir recording technique in such a manner as to produce recordings orbroadcasts which, when listened to with earphones, will have the desiredimproved qualities. However, this approach is of little value forpractical reasons. Firstly, it would establish an entirely new categoryof recording and broadcasting techniques, namely those which would beused with either earphone or loudspeaker listening specifically in mind.This is evidently undesirable because the result would be themanufacture of records and tapes, and the transmission of broadcastsfalling into one or the other of the two categories, with the listenerbeing compelled to listen in the selected way, or else settle for a veryconsiderable amount of distortion.

The use of this artificial hea method for program consumers is reallyout of the question right from the start, since .for the programconversion use must be made of a loudspeaker arrangement in order toapply to the artificial head the corresponding loudspeaker program.However, it is exactly this which should be avoided in many instances,because of the resulting disturbing noise and because of theconsiderably increased cost of the required equipment.

German Offenlegungsschrift 2,007,623 discloses an arrangement, notmaking use of an artificial human head, having the special purpose ofconverting electroacoustical stereophonic intensity information intoinformation modified to take into account sound trans mission time. Thisarrangement operates on the same principle as disclosed in US. Pat. No.3,088,977. It attempts to avoid the distortion in spatialcharacteristics associated with headphones, as compared to loudspeakers,through the use of delay circuitry and frequency-dependent dampingcircuitry for both earpieces of the earphone set and cupled to eachother. The coupling together is such as to deliberately introducecross-talk into the two channels, in an attempt to simulate thecross-talk in the acoustical perception of a listener listening tospaced stereo loudspeakers.

However, the electroacoustical transducer characteristics ofconventionally employed earphones vary markedly from one earphone typeto another. For this reason and others, such an arrangement is not veryeffective, even within the limits of the specific context and purposefor which it is intended, because the transducer characteristics of theearphones are in no sense taken into account. As a result, the knownarrangements do not make it possible to significantly avoid the spatialand spectral distortions in the subjective acoustical experience of theearphone user described above. Likewise, it is not possible with sucharrangements to simulate the specific spatial and spectral transducercharacteristics of loudspeakers, or of the particular loudspeaker towhich the listener is accustomed and which usually forms the basis ofwhat the listener subjectively judges to be an undistorted reproductionof sound. The creation of acoustical perceptions, which take intoaccount the characteristics of employed loudspeakers, can however forexample be necessary for a sound engineer, who in certain circumstancesmay be forced to monitor a broadcast or recording session usingearphones and will require and expect an exact correspondence betweenthe spectral and spatial characteristics of the sound he hears usingearphones and the sound he would hear if he employed the studioloudspeaker arrangement.

The general purpose of the invention is to overcome the disadvantages ofthe known methods and arrangements, and to provide a method which, withprogram material normally edited through the use of loudspeakers, suchas is the case with radio broadcasting, phonograph record recording, andtape recording, etc., can be listened to using earphones which produce asubjective acoustic experience corresponding as exactly as possible,both in spatial and spectral respects, to the acoustic experience of alistener listening to loudspeakers.

The invention exploits the surprising realization that it is possiblefor a person listening with earphones to have the subjective experienceof listening to loudspeakers, if only the physically measurable soundsignals impinging upon the listeners eardrums when listening withearphones are made to correspond as exactly as possible to the soundsignals impinging upon the eardrums when listening to loudspeakers. Thisrealization and basic approach are new. Hitherto it has always beentaken for granted that the problem in question could be solved only bytaking separately into account a sizable number of complicated andusually rather nebulous psychological and physiological considerations.

It is essential to the inventive concept that the sound signalsimpinging upon the listeners eardrums correspond as exactly as possibleto the sound signals which would impinge upon the listeners eardrums ifthe same audio information were transmitted to him from a loudspeakerarrangement. As explained before, the actual acoustical perception of alistener is a highly subjective matter and differs markedly form oneindividual to another. We have determined, after many series ofexperiments, that it is in fact possible to give the listener usingearphones the impression that he is listening to a loudspeakerarrangement simply through the duplication of the sound signalsimpinging upon the listener's eardrums when he listens to a loudspeaker.

In fact, experimentation has indicated that the basic principle of theinvention results in more than a mere improvement in the correspondencebetween the listening experience using earphones and that had whenlistening to loudspeakers; the resemblance between the two listeningexperiences has been made so extreme as to be astounding. In addition,the inventive expedient has proved realizable without any great increasein the usual costs of production.

According to the invention, it has been found advantageous to establisha close correspondence between the Fourier transform of the signalsimpinging upon the eardrums when the sound source is an earphone set andthe Fourier transform of the signals impinging upon the eardrums whenthe sound source is a loudspeaker arrangement. The Fourier transformsare advantageously made to correspond both with respect to magnitude andphase, and the result is a distortionless time shift of the soundsignals. However, a distortionless time shift,

i.e., a shift relative to the longer sound travel time when' usingloudspeakers, of the sound signals is of no consequence to the listener,provided that there is a sufficiently close correspondence between theFourier transforms of the signals with respect to magnitude and phase.In other words, the distance between the sound source and listener whena loudspeaker arrangement is employed can be ignored when attempting toduplicate that listening experience using only an earphone set.

The drawing depicts several embodiments of the method and arrangement ofthe invention. These exemplary embodiments deal with the problem ofimparting to earphones the sound-reproducing characteristics ofsymmetrical arrangements of one, two or four loudspeakers, in anenvironment substantially free of rever-' beration. v

FIG. 1 depicts in a very simplified and highly schematic manner thedifference between an objective acoustical event and a subjectiveacoustical perception;

FIGS. 2-5 depict a first exemplary embodiment of the invention in whichthe sound-reproducing characteristics of a single loudspeaker, disposedsymmetrically with respect to a listeners head, are to be imparted to anearphone arrangement;

FIGS. 6-8 depict a second embodiment, similar to that of FIGS. 25, butinvolving a pair of loudspeakers symmetrically disposed with respect tothe listeners head;

FIG. 9 depicts a third embodiment, similar to that of FIGS. 2-5, butinvolving four loudspeakers symmetrically disposed with respect to thelisteners head;

FIG. 10 depicts a test probe for insertion into the ear of a listener;

FIG. 11 isa diagram indicating the meaning of a number of differenttransfer functions, and showing the set-up for a particular test;

FIG. 12 depicts schematically an arrangement for measuring the frequencydependence of the magnitude of several transfer functions;

FIGS. 13 and 14 depict graphically the measurements made using thearrangement of FIG. 12;

FIG. 15 depicts schematically an arrangement for measuring the frequencydependence of the group delay time associated with several transferfunctions discussed with regard to the invention;

FIGS. 16 and 17 depict graphically the results of tests made using thearrangement of FIG. 15; and

FIG. 18 is a circuit diagram of a compensating stage I tion of the(n+2k) terminal device required for the simulation, n being the numberof loudspeakers and k the number of pairs of earphones. Theelectroacoustical transfer function A U) describes the relationshipexist- -ing between the electrical voltage U 0) applied to the inputof'the network and the signal pressures p U) and p ,(f) impinging uponthe eardrums of the listener, when the sound source is the singleloudspeaker oriented as shown in FIG. 2.

According to the basic object of the invention, ear- 7 phones are to beused to produce pressure signals impinging upon the listeners eardrumsand corresponding to p Q) and p U). To accomplish this, theelectro-acoustical transfer function of the earphones must firstbe.determined,as depicted schematically in FIG. 4. This furtherelectroacoustical transfer function is l(fl IPrmLfl/UKDI [pTRrm/ rm]This transfer function is that of the employed earphone pieces, and isequal to the ratio of the Fourier transforms of the driving voltageapplied to the earphone pieces and the output signal pressures impingingupon the left or right eardrum of the listener. The earphone is assumedto be a linear system. These transfer functions A U) and A can bedetermined, both with respect to magnitude and phase, through the use ofprobe-shaped microphones inserted into the ear of a listener and throughthe use of attenuation and phasemeasuring equipment. If one divides thetransfer function A U') by th transfer function A the circuit schemedepicted in FIG. 5 results, which can simultaneously serve to realize an(n+2k) terminal device. The signalpressures p ff) and 11 m in FIG. 5 arethe pressures impinging upon the listeners eardrums, when use is made ofa pair of earphones having respective electroacoustical transferfunctions A' (j) connected to the circuit stage X at junctions'Ban'd B.The transfer function [A (f)]- /[A (f)] can be realized by an electricalnetwork if one has determined the transfer function A Q) which dependsupon the earphones and the general configuration of the outer ear of ahuman head. In other words, when use is made of an arbitrarily selectedset of earphones, having a. known electroacoustical 7 transfer function,such earphones must be connected in the manner depicted in FIG. 5 to anetwork X having the transfer function indicated in FIG. 5. The networkX takes into account both the transfer function of the earphonesemployed and the transfer function of the loudspeaker arrangement to besimulated. Such a selection and connection of networks constitutes'theessential step for the achievement of a completely satisfactorylistening experience with earphones. In this way, and only in this way,will the subjective acoustical peprceptions of the listener using theearphone set correspond to those he would have when listening to theloudspeaker arrangement.

FIGS. 6-8 depict the embodiment wherein n=2, i.e., where thesound-reproducing characteristics to be simulated are those, of a pairof loudspeakers L and L disposed symmetrically with respect to thelisteners head. In this situation, .four transfer functions areinvolved,although because" of the symmetrical set-up the four transferfunctionsshould be composed of a first pair of identical transferfunctions and a second pair of identical transfer functions. The entiretransducer system can in the end be satisfactorily described by the twotransfer functions A 0) and A 0) and by the two summing 7 sures whenusing the loudspeaker arrangement of FIG.

FIG. 8 depicts, in block diagram form, a circuit arrangement forimparting to an earphone set-up the overall electroacoustical transducertransfer functions of the loudspeaker arrangement of FIG. 6. A (f) againidentifies the electroacoustic transfer function of the earphone setemployed in FIG. 4. The transfer functions A (f) and A are againdetermined by means of a probe microphone. With the exception of A' U),the

various transfer functions indicatedin FIG. and indicated in FIG. 8 canreadily be realized through the synthesis of appropriate input-outputnetworks using conventional techniques of transfer function synthesis.

The set of earphones with which one is to work, after theelectroacoustical transfer function A U') thereof has been plotted, willbe connected to the compensating network at connection points B, and B'as shown in FIG. 8. Self-evidently, more than one pair of earphones canbe connected to the outputs B, B ifthe number of pairs of earphones k isgreater than one.

FIG. 9 depicts a third embodiment, similar to the two just discussed,but intended to impart to a pair of earphones the sound-reproducingcharacteristics of a set of four loudspeakers disposed symmetricallywith respect to the head of a listener. Accordingly, n 4. The analysisof this circuit will be self-evident from the description of the twoprevious embodiments, except that two additional summing circuits S and8., are provided, in order to take into account the fact that each earreceives signals from all four of the loudspeakers.

,The determination of the transfer function A U) of the loudspeakerwhose sound-reproducing characteristics are to be simulated, andlikewise the determination of the transfer function A Q) of theearphones to be used in the simulation, can be achieved by employing aspecially designed microphone probe SM. This probe is depicted in FIG.10. It is comprised of a probe tube so configurated that when insertedinto the ear of a test listener the open end of the probe tube will belocated 4 mm behind the entrance into the lateral cartilaginous earcanal (as seen in the direction of the eardrum) in the reference planeBE, and it'will detect the pressure signals p arriving at that locationfrom the loudspeaker or from the earphones. The special form of theprobe tube guarantees that p can be measured even when the earphones arein place on the head of the listener. 1

FIG. 1 1 depicts schematically the head of the test listener VP (as seenfrom above), the outer ears OM of the listener, the ear canal GG whichterminates in the acoustical impedance Z ,(j) of the eardrum Tr, and theselected reference plane BE. The earphones are designated K, theloudspeaker designated L, the center of the listeners head M, the inputsignal to the loudspeaker designated U,. Also indicated area number oftransfer functions. These are as follows:

The above two-transfer functions are of interest with respect to thedetermination of the effective transfer function for the loudspeaker L.Also indicated are BEKU The above two transfer functions are of interestin determining the effective transfer functions for the earphones. Alsoindicated is 'rrsnm lPrrml [Pasml The above transfer function isassociated with the short length of the ear canal extending from theselected reference plane BE at which the pressure measurement isactually performed to the eardrum Tr itself. Given the impedance Z- U')of the eardrum, the ear canal transfer function A U) can readily bedetermined empirically. Clearly, the advantage of separately computingthe ear canal transfer function is that this procedure, which involvesan actual determination of the pressure variations at the eardrumitself, is a sensitive and potentially dangerous one. By separatelydetermining the transfer function A m, the pressure in the ear canalneed only be determined at the reference plane BE, at a distance of 4 mmfrom the ear canal entrance, avoiding the need to contact or extremelyclosely approach the eardrum. When the ear canal transfer function iscomputed, by empirical testing, it can be thereafter used incomputationsinvolving a variety of different loudspeaker and earphone transferfunctions. It should be noted that the separate determination of the earcanal transfer function A is not absolutely necessary. It would bepossible to insert the probe so deeply into the ear as to be spaced thesmallest possible distance from the eardrum itself. Evidently, howeverthat would be a less convenient and much more delicate procedure.

With these transfer functions computed, the transfer functions actuallyof interest can be easily computed. The loudspeaker transfer function AQ) will evidently be I um asum rraslfl The earphones transfer function AU') will evidently be M) ABEM rrsslf) The frequency dependence of themagnitude of the transfer function A (1') and of the transfer function AU) can be determined, for example, by employing an attenuationmeasurement unit such as depicted in FIG. 12. The probe SM is insertedinto the ear canal, both for the loudspeaker test and the earphone test.A conventional attenuation recorder 100, 101 is employed to determinethe shape of the curve of the magnitude of the transfer function versusfrequency. The generator generates a fixed-amplitude waveform, thefrequency ofwhich rises from zero linearly with time. This voltage isapplied to the input of either the earphone set or to the input of heloudspeaker L, depending upon which transfer function is to bedetermined. The pressure-responsive microphone SM responds to thepressure variations corresponding to the generated audio signals, andapplies a corresponding electrical signal to the input of an equalizingamplifier 102 which flattens the response curve of SM. The recorderdevice 100, 101 is provided with a chartering device 101 comprised of aroll of graph paper which is advanced in synchronism with the linearrise of frequency of the output voltage of the voltage generator 100.The device 101 includes a non-illustrated moving scribe whose positionvaries in dependence upon the output signal of amplifier 102. Theresulting curve inscribed upon the chart is a graph of thefrequencydependence of the magnitude of the measured transfer v functionA (f) or A Q). Examples of curves which have resulted from experimentsare shownin FIGS. 13

and 14.

FIG. I3 depicts in broken lines the frequency variation of the magnitudeof A U), the meaning of this transfer function having already beenexplained. The loudspeaker in question was spaced three meters from thecenter of the listeners head, and happened to be of the type marketedunder the designation ISOPHON KSB 12/8. The experiment was performed ina nonreverberating acoustical environment. The actual curve of interestis shown in solid lines and represents the frequency dependence of themagnitude of the transfer function A Q), defined above. The equationpermitting derivation of the-solid-line curve from the broken-line curvehas already been given.

FIG. 14 is similar to FIG. 13 but represents the test results when thecharacteristics of the earphone set were determined. The broken-linecurve BE. represents the magnitude of the transfer function A U), andthe solidlind curve designated Tr represents the 'magni-.

tude of the transfer function A Q'). The earphones employed for the testhappened to be of the type marketed under the designation BEYER DT 48.

As will be understood by persons skilled in the art,

the transfer functions in equation will exhibit frequency dependence notonly with respect to magnitude but also with respect to phase. It isaccordingly necessary to I determine ,the frequency dependenceof thephase sidered advantageous to determine the phaseshift indi rectly byinstead measuring the group delay time.

G m and G U) can be calculated, if for a particular reference frequencyfl, one measures the phase angles 9,,(fo) and G UZ) and then performsfrequencydependency measurements of 1, ,0) and 7 0).

The determination of 7, ,0) and of A!) is per se conventional, and can,by way of example, be performed with the arrangement depicted in FIG. 15(the measurement technique in question being known as the 10.:NyquistBrand technique). Theequations are as follows: 1

After these determinations, T,, (f) and 'r,, (f) can be calculated usingthe equations therefor given above. The function r flfl associated withthe path from reference plane BE to the eardrum can be determinedempirically, and is advantageously determined separately, for the samereason that the magnitude of the transfer function associated' 'with theterminal portion of the ear canal is determined separately, namely, toreduce the number of times that a probe is brought extremely near totheeardrum.

It is to be noted that instead of employing the equipment shown in FIG,15 to determine the frequen cy de pendence of the phase-shiftsassociated with the several transfer functions, it is possible to employthe conven-' delay time when determining the transfer, function A U).The curve designated BE representsthe group delay time r U) associatedwith the transfer func; tion A m wh ereas curve Tr represents the groupdelay time 1 m associated with the transfer function nm- The curvesdepicted in FIG. correspond to those depicted in FIG. 16, except aretaken with respect t o the earphones, instead of the loudspeaker.

-It is to be noted that the specific curves depicted represent averagesof the results of tests employing twelve different listeners. v I g vFIG. 18,.finally, depicts a circuit designed to have a transfer function[A (f)]/[A (f)] a and to, have the group delay time,r u,(f)/ xff)lariff) mm nearly as possible. The average delaytime difference amountstoabout 9 milliseconds,,inasmuch asno attempt wasrnade to take intoaccount the time required for the sound to travel the 3 meters frornthe.loudspeaker to the listenefis eardrum, because such distance and timedelay can be ignored with the inventive approach. v I

. Without further analysis, theforegoing will. so fully reveal the gistof the present invention that others can by applying current knowledgereadily adapt it for various' applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic orgspecific aspects of this inventionand, therefore, such modifications and adaptations should and areintended to be comprehended within the meaning and range of equivalenceof the following claims.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofcircuits and constructions different from the types described above.

While the invention has been illustrated and described as embodied in amethod and apparatus for im- 1 1 parting to earphones some of thesound-reproducing characteristics of loudspeakers, it is not to beconsidered limited to the details shown, since various modifications andstructural and circuit changes could be made without departing in anyway from the spirit of the invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1. Method for creating sound signals at the eardrums of a listener bymeans of earphones, to correspond to sound signals which would becreated at said eardrums of said listener in a predetermined acousticalenvironment in response to first electrical signals applied to aloudspeaker, comprising, in combination, the steps of measuring theamplitude and delay time of soundwaves at said eardrums of said listeneras a function of frequency in response to electrical signals applied tosaid loudspeaker when said listener is in said acoustical environment insuch a manner as to determine a corresponding desired transfer function;measuring the amplitude and delay time of soundwaves at said eardrums ofsaid listener as a function of frequency in response to electricalsignals applied to said earphones in such a manner as to determine acorresponding earphone transfer function; and furnishing an electricalnetwork having a network transfer function corresponding to the ratio ofsaid desired transfer function to said earphone transfer function, saidelectrical network having first input for receiving said firstelectrical signals and a pair of outputs; and connecting said earphonesto said pair'of outputs.

2. A method as set forth in claim 1, wherein a predetermined time shiftexists between soundwaves produces at said eardrums of said listener bysaid earphones and the soundwaves which would exist at said eardrums ofsaid listener in said predetermined acoustical environment in responseto said first electrical signals.

3. Arrangement for creating sound signals at the eardrums of a listenerto correspond to sound signals which would be created at said eardrumsof said listener in response to electrical-signals applied to aloudspeaker positioned symmetrically to said eardrums of said listener,and wherein a transfer function A U') defines the sound pressure at saideardrums of said listener in response to predetermined first electricalsignals applied to said loudspeaker, comprising, in combination,earphone means for creating an earphone transfer characteristic A saidearphone transfer characteristic constituting a measure of the variationwith respect to frequency of the sound pressure at said eardrums inresponse to earphone test signals applied to said earphones; networkmeans having at least one input for receiving electrical signals, a pairof outputs, and a network transfer function corresponding to the ratioof A divided by A and connecting means for connecting said earphones tosaid pair of outputs of said network means and said first electricalsignals to said input of said network means.

4. An arrangement as set forth in claim 3, wherein said predeterminedacoustical environment comprises a first and second loudspeakerpositioned symmetrically to said eardrums of said listener andresponsive, respectively, to first and second electrical signals;wherein a first and second desired transfer function A Q) and A 0)define, respectively, the transfer function from the input of said firstloudspeaker to said first and second eardrum of said listenerrespectively; wherein said network means have a first and second inputfor, respectively, receiving said first and second electrical signals;wherein said electrical network means comprise a first and secondsumming stage, each of said summing stages having a first and secondinput and a summing output; wherein said electrical network meansfurther comprise a first and second input stage having a transferfunction substantially equal to A 0) divided by A 0) and a first andsecond output stage having a transfer function substantially equal to AU') divided by A U'); wherein said network means further comprise firstconnecting means connecting said first input of said network meansdirectly to said first input of said first summing stage, and saidsecond input of said network means directly to said first input of saidsecond summing stage, second connecting means for interconnecting saidfirst input stage between said first input of said network means andsaid second input of said second summing stage; third connecting meansfor connecting said second input stage between said second input of saidnetwork means and said second input of said first summing stage; andfourth connecting means connecting said first output stage between saidsumming output of said first summing amplifier and said first output ofsaid network means and for connecting said second output stage betweensaid output of said second summing stage and said second output of saidnetwork means.

5. An arrangement as set forth in claim 4, wherein said predeterminedacoustical environment further comprises a third and fourth loudspeaker,arranged symmetrically with respect to said eardrums of said listener;further comprising additional network means, corresponding to saidnetwork means set forth in claim 4, and connected to said third andfourth loudspeaker, said additional network means also having a pair ofoutputs; further comprising a first and second additional summing stageeach having a first and second input and an output; further comprisingmeans for connecting one of said pairs of outputs of said first andadditional network means respectively to said first and second input ofsaid first additional summing stage, and the other of said outputs ofsaid first and additional network means to said first and second inputsof said second additional summing stage; and means for connecting saidfirst and second earphones respectively to said output of said first andsecond additional summing stages.

1. Method for creating sound signals at the eardrums of a listener bymeans of earphones, to correspond to sound signals which would becreated at said eardrums of said listener in a predetermined acousticalenvironment in response to first electrical signals applied to aloudspeaker, comprising, in combination, the steps of measuring theamplitude and delay time of soundwaves at said eardrums of said listeneras a function of frequency in response to electrical signals applied tosaid loudspeaker when said listener is in said acoustical environment insuch a manner as to determine a corresponding desired transfer function;measuring the amplitude and delay time of soundwaves at said eardrums ofsaid listener as a function of frequency in response to electricalsignals applied to said earphones in such a manner as to determine acorresponding earphone transfer function; and furnishing an electricalnetwork having a network transfer function corresponding to the ratio ofsaid desired transfer function to said earphone transfer function, saidelectrical network having first input for receiving said firstelectrical signals and a pair of outputs; and connecting said earphonesto said pair of outputs.
 2. A method as set forth in claim 1, wherein apredetermined time shift exists between soundwaves produces at saideardrums of said listener by said earphones and the soundwaves whichwould exist at said eardrums of said listener in said predeterminedacoustical environment in response to said first electrical signals. 3.Arrangement for creating sound signals at the eardrums of a listener tocorrespond to sound signals which would be created at said eardrums ofsaid listener in response to electrical signals applied to a loudspeakErpositioned symmetrically to said eardrums of said listener, and whereina transfer function AM(f) defines the sound pressure at said eardrums ofsaid listener in response to predetermined first electrical signalsapplied to said loudspeaker, comprising, in combination, earphone meansfor creating an earphone transfer characteristic AK, said earphonetransfer characteristic constituting a measure of the variation withrespect to frequency of the sound pressure at said eardrums in responseto earphone test signals applied to said earphones; network means havingat least one input for receiving electrical signals, a pair of outputs,and a network transfer function corresponding to the ratio of AM(f)divided by AK(f); and connecting means for connecting said earphones tosaid pair of outputs of said network means and said first electricalsignals to said input of said network means.
 4. An arrangement as setforth in claim 3, wherein said predetermined acoustical environmentcomprises a first and second loudspeaker positioned symmetrically tosaid eardrums of said listener and responsive, respectively, to firstand second electrical signals; wherein a first and second desiredtransfer function AC(f) and AS(f) define, respectively, the transferfunction from the input of said first loudspeaker to said first andsecond eardrum of said listener respectively; wherein said network meanshave a first and second input for, respectively, receiving said firstand second electrical signals; wherein said electrical network meanscomprise a first and second summing stage, each of said summing stageshaving a first and second input and a summing output; wherein saidelectrical network means further comprise a first and second input stagehaving a transfer function substantially equal to AC(f) divided by AS(f)and a first and second output stage having a transfer functionsubstantially equal to AS(f) divided by AK(f); wherein said networkmeans further comprise first connecting means connecting said firstinput of said network means directly to said first input of said firstsumming stage, and said second input of said network means directly tosaid first input of said second summing stage, second connecting meansfor interconnecting said first input stage between said first input ofsaid network means and said second input of said second summing stage;third connecting means for connecting said second input stage betweensaid second input of said network means and said second input of saidfirst summing stage; and fourth connecting means connecting said firstoutput stage between said summing output of said first summing amplifierand said first output of said network means and for connecting saidsecond output stage between said output of said second summing stage andsaid second output of said network means.
 5. An arrangement as set forthin claim 4, wherein said predetermined acoustical environment furthercomprises a third and fourth loudspeaker, arranged symmetrically withrespect to said eardrums of said listener; further comprising additionalnetwork means, corresponding to said network means set forth in claim 4,and connected to said third and fourth loudspeaker, said additionalnetwork means also having a pair of outputs; further comprising a firstand second additional summing stage each having a first and second inputand an output; further comprising means for connecting one of said pairsof outputs of said first and additional network means respectively tosaid first and second input of said first additional summing stage, andthe other of said outputs of said first and additional network means tosaid first and second inputs of said second additional summing stage;and means for connecting said first and second earphones respectively tosaid output of said first and second additional summing stages.